The present invention relates to devices for interacting with biological species; and more particularly, it relates to devices for physically interacting with charged species via electrostatic forces.
It has been demonstrated in the art that attractive and repulsive electrostatic forces underlie the initial interactions between biological cells and a well-defined surface. Gingell and Fornes in "Interaction of Red Blood Cells with a Polarized Electrode", Biophysical Journal, Vol. 16, page 1131 (1976), discuss the adhesion of glutaraldehyde--fixed human red blood cells to a smooth polarizable lead electrode in dilute sodium fluoride solution. The use of electrostatic interaction between chemical species to hold cells has been commercially implemented by Bio-Rad Laboratories, 2200 Wright Avenue, Richmond, Calif. 94804 in its BIO-CARRIERS.RTM. microcarrier cell culture to hold cells. No manipulative control over cells has been provided, however.
Methodology for separating charged macromolecules and particulate species, including biological entities, to a large extent has its basis in electrostatic interactions, for example, electrical field-flow fractionation (FFF) which utilizes electrical potential differences. FFF is an elution method relying on the fractionation of solutes while moving through a channel. As the carrier stream moves through the channel, an external electrostatic force field is applied at right angles to the flow. The field interacts with the solute and forces it against the channel wall forming a layer. The mean thickness of the layer differs for each distinct chemical or particulate species, thereby providing a basis for selective retention, and produces an elution spectrum.
Another method for separation of charged species is electrophoresis. In this method, charged molecules are moved through a suitable medium by an electric field which permeates the medium. The molecules are separated on the basis of different electrophoretic mobilities.
It is also known to separate molecules according to charge using ion exchange chromatography. In this method, a column packing is used which consists of an insoluble support, charged functional groups covalently bound to the support, and mobile counterions which associate with the functional groups because of an opposite charge. When a mixture is passed through the column, molecules of neutral or like charge to the functional groups are eluted while oppositely charged molecules compete with the counterions for binding sites on the functional groups. Molecules more highly charged than the counterions become bound to the matrix and are retained on the column. An eluant having an appropriate ionic strength and pH is used to recover the bound sample. An example of how a variable system can greatly augment the sensitivity of such separations is that of isoelectric focusing. In this technique, the electrophoretic differences between compounds are coupled to their inherent differences in the pH at which their electrical charges are neutralized.
The existing techniques and devices based on electrostatic interaction with charged species do not offer the capabilities of continuous and dynamic control of the electrostatic field. That is, electrostatic interaction in these processes is neither continuously variable nor reversible. Moreover, the known techniques do not lend themselves to practical implementation in a small volume instrument. Further, due to the physical contact of both electrodes with a conducting medium, significant current flow occurs during the generation of an electrostatic field. Although electrostatic interaction with biological cells has been applied as a mechanism for the manipulation of cells, such as cell sorting, this approach due to the above-noted limitations has yet to be fully exploited, particularly on the level of an individual cell.